The detection of underground pipes and utility lines is a necessity when any construction work involving digging trenches is necessary. Many types of service lines such as telephone and telegraph cables, gas and sewer lines, electrical lines and water mains are located below the surface. It has become increasingly popular to utilize plastic pipe or plastic wrapped cables in this service. Also, ceramic and other nonmetallic materials such as concrete are used in underground service. The precise location of the nonmetallic lines cannot be determined by the metal detectors above ground used to locate metal pipe. It is important to have a simple method to locate these underground lines when underground construction is in the vicinity or repair of the lines is needed.
This problem has been addressed in several patents. In U.S. Pat. No. 3,115,861 a frangible colored body is laid underground over the buried service line. The colored body has a water insoluble color and has the benefit of being visible when contacted with soil during digging operations to warn the operator of the proximity of a service line. In U.S. Pat. No. 3,282,057 the use of a colored plastic tape laid over a service line when it was installed was disclosed. The plastic tape is colored and when it is pulled up by a digger, it provides a warning of the proximity of a buried line. Both patents have the drawback that the colored frangible bodies or tapes are not detectable from the surface. Also, the use of excavation equipment which makes deep entrenchments may encounter the colored body or tape. and the line in one operation which would provide no warning.
In U.S. Pat. No. 3,504,503 a method was disclosed for locating underground lines of the construction discussed herein which included using a metallic foil with a moisture and soil resistant coating and a coloring to distinguish from the soil. The colored foil frangible body is laid over the service line. The benefit of the foil is that metal detectors can be used to locate the foil and thereby the location of the service line.
In U.S. Pat. No. 3,633,533, a method to locate underground service lines is disclosed using a metal film coated with a colored plastic. The plastic is moisture and soil resistant. In an alternative embodiment a metallic wire is under the plastic coating. The plastic coated foil tape is used quite extensively. There are color codes for water, gas, electric, buried telephone and telegraph lines and other services. The tapes are printed with a cautionary message corresponding to the type of service line underlying the tape. With the proliferation of plastic pipe the tapes have widespread use throughout the world.
The plastic coated metallic tapes have a life of about six to seven years. The life of the tape depends upon the moisture and soil conditions which will invade the edges of the tape where the metal is exposed and cause corrosion of the metallic element. There are certain bacterial conditions which contribute to an environment which will cause corrosion of the metallic element and deterioration of the pigment. Also, many of the tapes have printed messages on the outside of the tape which will wear off with abrasion.
The plastic coated tapes in use have other drawbacks. When a trench is dug for a utility line and a detectable tape is used, typically, the line is laid first. Then there is a partial backfilling of the trench and the tape is laid over the line coming off a roll on a tractor. Then in a continuous process the trench backfilling is completed. As the tape is laid there may be some tension placed on it, as the backfill process is completed, stretching the tape. During this process if too much tension is placed on the tape, it can break causing the operation to be halted until the tape can be mended. Also, the foil layer is not elastic and tension on the tape may cause the metallic foil to break while the plastic coatings are stretched. In some cases a continuous length of metallic conductive material is desirable. Also, the present plastic coated types have a tendency to curl on the edges and care must be taken when laying the tape that it does not become twisted or folded. Although the plastic coated metallic foil tapes have provided a cost efficient method to detect underground piping systems, there are some aspects which can be improved upon.